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Congestion Control. Outline Queuing Discipline Reacting to Congestion Avoiding Congestion. Source. 1. 10-Mbps Ethernet. Router. Destination. 1.5-Mbps T1 link. 100-Mbps FDDI. Source. 2. Issues. Two sides of the same coin pre-allocate resources so at to avoid congestion - PowerPoint PPT Presentation
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1
Congestion Control
OutlineQueuing Discipline
Reacting to Congestion
Avoiding Congestion
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Issues• Two sides of the same coin
– pre-allocate resources so at to avoid congestion– control congestion if (and when) is occurs
• Two points of implementation– hosts at the edges of the network (transport protocol)– routers inside the network (queuing discipline)
• Underlying service model– best-effort (assume for now)– multiple qualities of service (later)
Destination1.5-Mbps T1 link
Router
Source2
Source1
100-Mbps FDDI
10-Mbps Ethernet
3
Framework• Connectionless flows
– sequence of packets sent between source/destination pair– maintain soft state at the routers
• Taxonomy– router-centric versus host-centric– reservation-based versus feedback-based– window-based versus rate-based
Router
Source2
Source1
Source3
Router
Router
Destination2
Destination1
4
Evaluation
• Fairness• Power (ratio of throughput to delay)
Optimalload Load
Th
rou
ghp
ut/d
elay
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Queuing Discipline• First-In-First-Out (FIFO)
– does not discriminate between traffic sources• Fair Queuing (FQ)
– explicitly segregates traffic based on flows– ensures no flow captures more than its share of capacity– variation: weighted fair queuing (WFQ)
• Problem?Flow 1
Flow 2
Flow 3
Flow 4
Round-robinservice
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FQ Algorithm
• Suppose clock ticks each time a bit is transmitted• Let Pi denote the length of packet i• Let Si denote the time when start to transmit packet i• Let Fi denote the time when finish transmitting packet i• Fi = Si + Pi
• When does router start transmitting packet i?– if before router finished packet i - 1 from this flow, then
immediately after last bit of i - 1 (Fi-1)– if no current packets for this flow, then start transmitting
when arrives (call this Ai)• Thus: Fi = MAX (Fi - 1, Ai) + Pi
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FQ Algorithm (cont)
• For multiple flows– calculate Fi for each packet that arrives on each flow– treat all Fi’s as timestamps– next packet to transmit is one with lowest timestamp
• Not perfect: can’t preempt current packet• Example
Flow 1 Flow 2
(a) (b)
Output Output
F = 8 F = 10F = 5
F = 10
F = 2
Flow 1(arriving)
Flow 2(transmitting)
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TCP Congestion Control
• Idea– assumes best-effort network (FIFO or FQ routers)each
source determines network capacity for itself
– uses implicit feedback
– ACKs pace transmission (self-clocking)
• Challenge– determining the available capacity in the first place
– adjusting to changes in the available capacity
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Additive Increase/Multiplicative Decrease
• Objective: adjust to changes in the available capacity• New state variable per connection: CongestionWindow
– limits how much data source has in transit
MaxWin = MIN(CongestionWindow, AdvertisedWindow)
EffWin = MaxWin - (LastByteSent - LastByteAcked)
• Idea:– increase CongestionWindow when congestion goes down– decrease CongestionWindow when congestion goes up
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AIMD (cont)
• Question: how does the source determine whether or not the network is congested?
• Answer: a timeout occurs– timeout signals that a packet was lost– packets are seldom lost due to transmission error– lost packet implies congestion
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AIMD (cont)
• In practice: increment a little for each ACKIncrement = (MSS * MSS)/CongestionWindow
CongestionWindow += Increment
Source Destination
…
• Algorithm– increment CongestionWindow by
one packet per RTT (linear increase)
– divide CongestionWindow by two whenever a timeout occurs (multiplicative decrease)
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AIMD (cont)
• Trace: sawtooth behavior
60
20
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0
KB
Time (seconds)
70
304050
10
10.0
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Slow Start
• Objective: determine the available capacity in the first
• Idea:– begin with CongestionWindow = 1
packet– double CongestionWindow each RTT
(increment by 1 packet for each ACK)
Source Destination
…
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Slow Start (cont)• Exponential growth, but slower than all at once• Used…
– when first starting connection– when connection goes dead waiting for timeout
• Trace
• Problem: lose up to half a CongestionWindow’s worth of data
60
20
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0
KB
70
304050
10
15
Fast Retransmit and Fast Recovery
• Problem: coarse-grain TCP timeouts lead to idle periods
• Fast retransmit: use duplicate ACKs to trigger retransmission
Packet 1
Packet 2
Packet 3
Packet 4
Packet 5
Packet 6
Retransmitpacket 3
ACK 1
ACK 2
ACK 2
ACK 2
ACK 6
ACK 2
Sender Receiver
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Results
• Fast recovery– skip the slow start phase– go directly to half the last successful CongestionWindow (ssthresh)
60
20
1.0 2.0 3.0 4.0 5.0 6.0 7.0
KB
70
304050
10
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Congestion Avoidance• TCP’s strategy
– control congestion once it happens
– repeatedly increase load in an effort to find the point at which congestion occurs, and then back off
• Alternative strategy– predict when congestion is about to happen
– reduce rate before packets start being discarded
– call this congestion avoidance, instead of congestion control
• Two possibilities – router-centric: DECbit and RED Gateways
– host-centric: TCP Vegas
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DECbit• Add binary congestion but to each packet header• Router
– monitors average queue length over last busy+idle cycle
– set congestion bit if average queue length > 1– attempts to balance throughout against delay
Queue length
Currenttime
TimeCurrent
cyclePrevious
cycleAveraginginterval
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End Hosts
• Destination echoes bit back to source• Source records how many packets resulted in set bit• If less than 50% of last window’s worth had bit set
– increase CongestionWindow by 1 packet
• If 50% or more of last window’s worth had bit set – decrease CongestionWindow by 0.875 times
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Random Early Detection (RED)
• Notification is implicit – just drop the packet (TCP will timeout)– could make explicit by marking the packet
• Early random drop– rather than wait for queue to become full, drop each
arriving packet with some drop probability whenever the queue length exceeds some drop level
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RED Details• Compute average queue length
AvgLen = (1 - Weight) * AvgLen + Weight * SampleLen
0 < Weight < 1 (usually 0.002)SampleLen is queue length each time a packet arrives
MaxThreshold MinThreshold
AvgLen
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RED Details (cont)
• Two queue length thresholds
if AvgLen <= MinThreshold then
enqueue the packet
if MinThreshold < AvgLen < MaxThreshold then
calculate probability P
drop arriving packet with probability P
if ManThreshold <= AvgLen then
drop arriving packet
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RED Details (cont)• Computing probability P
TempP = MaxP * (AvgLen - MinThreshold)/ (MaxThreshold - MinThreshold)
P = TempP/(1 - count * TempP)
• Drop Probability CurveP(drop)
1.0
MaxP
MinThresh MaxThresh
AvgLen
24
Tuning RED• Probability of dropping a particular flow’s packet(s) is roughly
proportional to the share of the bandwidth that flow is currently getting• MaxP is typically set to 0.02, meaning that when the average queue size
is halfway between the two thresholds, the gateway drops roughly one out of 50 packets.
• If traffic id bursty, then MinThreshold should be sufficiently large to allow link utilization to be maintained at an acceptably high level
• Difference between two thresholds should be larger than the typical increase in the calculated average queue length in one RTT; setting MaxThreshold to twice MinThreshold is reasonable for traffic on today’s Internet
• Penalty Box for Offenders
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TCP Vegas• Idea: source watches for some sign that router’s queue is
building up and congestion will happen too; e.g.,– RTT grows
– sending rate flattens 60
20
0.5 1.0 1.5 4.0 4.5 6.5 8.0
KB
Time (seconds)
Time (seconds)
70
304050
10
2.0 2.5 3.0 3.5 5.0 5.5 6.0 7.0 7.5 8.5
900
300100
0.5 1.0 1.5 4.0 4.5 6.5 8.0
Sen
ding
KB
ps
1100
500700
2.0 2.5 3.0 3.5 5.0 5.5 6.0 7.0 7.5 8.5
Time (seconds)0.5 1.0 1.5 4.0 4.5 6.5 8.0Q
ueue
siz
e in
rou
ter
5
10
2.0 2.5 3.0 3.5 5.0 5.5 6.0 7.0 7.5 8.5
26
Algorithm • Let BaseRTT be the minimum of all measured RTTs (commonly the
RTT of the first packet)• If not overflowing the connection, then
ExpectRate = CongestionWindow/BaseRTT• Source calculates sending rate (ActualRate) once per RTT• Source compares ActualRate with ExpectRate
Diff = ExpectedRate - ActualRateif Diff <
increase CongestionWindow linearlyelse if Diff >
decrease CongestionWindow linearlyelse
leave CongestionWindow unchanged
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Algorithm (cont)
• Parameters = 1 packet = 3 packets
• Even faster retransmit– keep fine-grained timestamps for each packet – check for timeout on first duplicate ACK
70605040302010
KB
Time (seconds)
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0
CA
M K
Bps
240200160120
8040
Time (seconds)